Process for the ultrapurification of alginates

ABSTRACT

A method for obtaining solutions of non-structurally modified salts of alginate, with endotoxin content less than 20 EU/g includes obtaining an alginate solution having a concentration ranging from 1.6 to 2.0% by weight by adding commercial grade alginate powder to a saline solution. The alginate solution obtained is filtered on at least one hydrophilic filter, and a filtered alginate solution is recovered. The filtered alginate solution obtained is further filtered on a charge-modified hydrophobic filter, and an alginate solution having an endotoxin content less than 20 EU/g is recovered.

TECHNICAL FIELD

The present invention relates to a process for the ultrapurification ofalginates, in particular intended for microencapsulation in human celltransplants. The process is advantageously applied to the purificationof the starting powder of pharmaceutical grade sodium alginate, removingendotoxins and endogenous pyrogens, yet keeping unchanged the molecularstructure of the product.

RELATED ART

Sodium alginate (AG) is a polysaccharide extracted from some seaweeds,in particular Macrocystis pyrifera, present above all along the westerncoasts of the Pacific, which finds wide use in various fields,biotechnology included, The biopolymer salts, whose purification issubject-matter of the present invention, are water-solublepolysaccharides that are either spontaneously exuded by, or areextracted from living vegetal organisms. In fact, alginates are salts ofalginic acid, which shows a copolymer structure Dmannuronic acid (-M-),and -L-guluronic acid (-G-) units), These units form polymer or dimerblocks MM or GG that at times alternate in the molecular pattern.Alginates molecular arrangement and composition are determined primarilyby the source from which they are obtained. For example, the mostcommonly used alginates derive from brown seaweeds, and in particularproducts deriving from Macrocystis pyrifera have an M/G ratio equal to1.56:1, while those from Laminaria Hyperborea have an M/G ratio equal to0.45. Monovalent salts (Na, K) of alginate are typically water soluble,unlike divalent salts (Ba, Ca) or polyvalent salts (Fe, Al) that arefound in the form of gels or solids.

AG has been used for years in food and pharmaceutical industries for theproduction, respectively, of fruit jellies and excipients for someclasses of drugs (e.g., antiacids, etc.). However, commerciallyavailable sodium alginate is not sufficiently purified for specialapplications, such as applications in human transplants, where strictand internationally recognized quality control criteria are required,e.g. the guidelines of the Ministry of Health or those of U.S.Pharmacopeia.

Alginate is commercially available both as raw extract to be purifiedand as partially purified solution. The chemical composition of alginatepowder is described in terms of fractions (F_(G) or F_(M)) and M/Gratio. The product AG KELTONE LVCR, for example, has an endotoxin levelin the range from about 30,000 EU/g to about 60,000 EU/g that, as such,makes it unsuitable for parenteral applications requiring an endotoxinlevel of no more than 100 EU/g, though lower levels are preferable.

As a consequence of this, and before AG KELTONE LVCR may be usedparenterally, the level of endotoxin must be drastically reduced.

For over 20 years, AG has been employed in the preparation ofmicrocapsules containing hybridoma cells for the production ofmonoclonal antibodies (Damon Biotech, Inc.) as well as for protection ofpancreatic islands (insulae) transplants from host rejection reaction(U.S. Pat. No. 4,683,092). The first microcapsular transplant protocolsin diabetic rodents and higher mammals clearly highlighted purity ofemployed material to be fundamental for success of the transplantitself. The main contaminant, possibly responsible for microcapsulartransplant failure, is represented by bacterial lipopolysaccharideendotoxins, pyrogenic materials present in the membrane of gram negativebacteria. Such substances are resistant to most sterilization systems(e.g., autoclave). Techniques such as gamma irradiation or dry heatsterilization are able to destroy endotoxins, but can damage thematerials or products to be sterilized. Moreover, their generally lowmolecular weight (10-20 Kd) does not allow removal thereof by standardfiltration processes. In any case, it should be taken into account theneed to obtain a sterile, as well as endotoxin-free product, in order toavoid the risk of secondary contamination. Lastly, materials which areto be introduced parenterally in the human body must have an endotoxincontent lower than 100 EU/g, though it would be preferable to have alevel lower than 50 EU/g. In case of AG use for the preparation ofmicrocapsules containing pancreatic islands, for transplant purposes,endotoxin presence may invalidate capsule-provided immunoprotection,fostering the onset of a serious inflammatory reaction.

The need to obtain an ultrapurified product (virtually “endotoxin-free”)has fostered in the last years the development of some purificationmethods, which however are unable to meet all industrial and safetydemands. Some purification methods envisaging the W use of ion exchangeresins with or without addition of polymyxin-b or filtration oncellulose acetate filters and further membrane dialization had thedrawbacks of allowing no relevant reduction of endotoxin levels (about70-80 EU/g), being excessive in cost and not applicable to theproduction of bulk quantities of AG owing to a remarkable loss of massof the starting product, virtually indispensable to the start ofprotocols for clinical use, and, in the case of chloroform use, ofexhibiting a difficult removal of said solvent, potentially toxic evenin modest amounts. For example, in the technique employing AGprecipitation in ethanol and subsequent extraction with chloroform, inorder to obtain 1 liter of end product it is necessary to start fromabout 10 liters of AG. Moreover, a low yield of the product implies thatdifferent AG batches may have non-homogeneous features, certainlyunsuitable for preparing products to be used in clinical protocols, forwhich technologies reproducible on a large scale are essential. U.S.Pat. No. 6,451,772, starting from raw alginate, substantially providesfiltration (and/or use of ion exchange resins) on polypropylene filtersand subsequent filtrate precipitation with organic solvents. Mainlimitations of this process are represented by

-   -   1. the excessive cost of the materials, with respect to the end        volumetric yield in terms of alginate produced, yield which        however remains unsatisfactory,    -   2. the fact that the described methodology has excessive        variability and does not allow the systematic availability of        the product itself, though occasionally alginate with a        sufficiently low endotoxin content is obtained; and    -   3. the modification effect on alginate structure caused by        solvent use.

SUMMARY

It was now surprisingly found that replacing the precipitation operationwith a filtration on a cartridge with charge-modified filtrationmembrane allows to obtain AG

-   -   1. of very high and constant purity degree (endotoxin content        ≦20 EU/g)    -   2. ultrapurified on a large scale,    -   3. obtained without use of solvent, and therefore        -   a. without modification of the chemical structure of            alginate        -   b. and without alginate contamination, so as to be accepted            by protocols for parenteral use.

It is therefore subject-matter of the present invention a process forthe obtainment of solutions of salts of alginate not structurallymodified, with endotoxin content not higher than 20 EU/g, comprising thefollowing steps:

-   -   a. addition of commercial grade alginate powder to a saline        solution, until obtaining an alginate solution having a        concentration ranging from 1.6 to 2.0% by weight and pH        adjustment in the range 7.4-7.6    -   b. filtration of the solution obtained from step a) on at least        one hydrophilic filter and recovery of the solution obtained

characterized in that the solution obtained from step b) is subjected tofiltration on a hydrophobic filter and the obtained solution isrecovered.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

According to the invention, an alginate used is sodium alginate having acomposition comprising 52.26% M and 47.74% G, corresponding to an M/Gratio of 1.093. Advantageously, as alginate the product Keltone® LVCRwas used.

The filtration of step b) preferably occurs by using three filters ofcellulose acetate hydrophils, of which advantageously the first one hasa pharmaceutical grade 30 and a nominal pore size of 2 μm, the secondone a pharmaceutical grade 60 and a nominal pore size of 2 μm, and thethird one a pharmaceutical grade 90 and a nominal pore size of 2 μm.

The second filtration makes use of a hydrophobic filter; it isadvantageously preferred a charge-modified nylon filter, in particular anylon 66 filter having a positive electric charge.

According to the invention, from the pharmaceutical grade AG powder,after molar dilution and multiple filtration, an end product is obtained(both in the form of solution and powder) whose endotoxin content doesnot exceed 20 EU,/g, in full compliance with the above-stated qualitycontrol criteria. The end product of the present inventors, usuallyavailable in 1.8% (w/v) solution, appropriately stored in alight-protected environment and at a temperature of 4°-6° C., is stableover time for about 5 years with a virtually absent protein content(<0.4%—another U.S. FDA bioinvisibility standard).

The invention exploits the chemical structure of lipopolysaccharideendotoxins (LUPS) consisting of two portions, one hydrophilic(polysaccharide) and one hydrophobic (lipid). According to theinvention, at the end of the ultrafiltration process (positivelycharged) nylon filters were employed, able to selectively bind the(hydrophobic) lipid portion of endotoxins, retaining them withoutaltering and/or damaging AG structure. Adding the fourth filter, besidesobviating further manipulations, required according to the prior art,guarantees a reproducible and simple result. Moreover, the chemicalstructure of the product is not altered, an aspect that had beenhighlighted in the above-mentioned U.S. patent. This is incontrovertiblydemonstrated by NMR spectra of the product showing, both in the case ofDeuterium and of Carbon, that there are no structural nor molecularvariations, not even minimal ones, between the (unpurified) startingproduct and the (ultrapurified, clinical grade) end product.

The process according to the invention allows obtaining about 50% of thestarting product, in comparison with the 10% that may be calculated asrecovered amount with the other methodologies.

The process of the present invention is simple to carry out, withlimited manual interventions, therefore less at risk of contaminationthan the previous known ones, as it entails only the assembly of thesuitable filter in the sanitary container (“housing”) and the connectionto the pumping system, down to the collecting one. The entiremethodology may easily be conducted under a class II laminar-flowbiological hood. Protein content of these samples ranges from 0 to 0.016mg/ml. These data were calculated on values obtained in 21 differentfiltration processes accomplished in 2 years, as shown in the followingTable 1.

Table 1

Protein content in various samples obtained according to the process ofthe invention Average: 0.090846

Purification Proteins (internal number) (mq/ml) 1 0.0072 2 0.0087 30.0179 4 0.0095 5 0.0116 6 0.0121 7 0.0077 8 0.0062 9 0.0098 10 0.011811 0.0125 12 0.0124 13 0.0138 14 0.0089 15 0.0120 16 0.0131 17 0.0088 180.0063 19 0.0110 20 0.0157 21 0.0079

The process of the invention comprises two steps:

-   -   1. Dissolving the AG powder in sodium chloride, controlling the        pH, and passing through three hydrophilic filters.        -   Preferably, as hydrophilic filtering material it is employed            cellulose acetate, or other commercially available filters,            for example “zetaplus” grade Cuno filters having a pore size            ranging from 1 microns to 0.1 microns, with a wide filtering            surface such as that employed by the present inventors,            which offers the advantage of reducing filtration pressure            without altering the product and of removing cell fragments            and the so-called “inert” microparticles. The product thus            obtained is sterile, with an endotoxin content anyhow higher            than 100 EU/g;    -   2. the second step, supporting its originality, is the removing        of residual endotoxins by a hydrophobic nylon filter having a        positive electric charge, in order to bind the negative portion        of the lipopolysaccharide preventing precipitation on solvent or        of other techniques, obtaining a product ready for various uses,        having an endotoxin content always lower than 30 EU/g.

The present invention finds application in the field of transplantationbiotechnologies, in particular in the production of AG microcapsulessuitably coated with polyamino acids and diluted AG, which weredemonstrated to immunoprotect transplants of islands from host immunesystem cells. For several years now the inventors have been studying thetransplant of microencapsulated pancreatic islands for therapy of type 1diabetes mellitus (insulin-dependent, or T1DM) and this researchactivity is amply documented by scientific publications, mostlyinternational ones. Moreover, thanks to the purity and highbiocompatibility of the materials employed, the present inventors havebeen authorized by the Italian Istituto Superiore di Sanita tooperatively begin a phase I clinical study related to transplant ofmicroencapsulated human pancreatic islands in patients with T1DM, notpharmacologically immunosuppressed. From results obtained hereto itemerges that the microcapsules obtainable according to the invention arehighly biocompatible, and the effectiveness of their product has beenassessed by various international research laboratories.

EXAMPLES

For the preparation of the solution of sodium alginate, ultrapure andhaving an endotoxin level no higher than 20 EU/g, the followingmaterials are needed:

-   -   Pyrex beaker,    -   Pyrex graduated cylinders,    -   Magnetic stirrer,    -   Silicone tube HW 155 inner diameter 5, outer diameter 8,    -   Peristaltic pump,    -   Sterile pipettes,    -   Certified endotoxin-free sterile bottles,    -   30, 60 and 90 pharmaceutical grade cellulose acetate filters        with filtration coadjuvant comprised of diatomite and perlite,        20″ height and 12″ diameter    -   Filters with charge-modified Nylon 66 filtration membrane, pore        size 0.2 μm, 20″ height and 8″ diameter.    -   AISI316 stainless steel sanitary container for 20″ filtration        cartridge, 12″ diameter.    -   AISI316 stainless steel sanitary container for 20″ filtration        cartridge, 8″ diameter.

The methodology of preparation of the solution of sodium alginate,ultrapure and with an endotoxin level no higher than 20 EU/g providesthe following steps:

Sterilization of “Housings”, Filters and Material of Use

Glass containers (graduated beaker, graduated cylinder, bottles, etc.),connecting silicone tubes, AISI316 stainless steel sanitary containers,referred to as “Housings”, and any other material (magnetic anchors,glass rods, etc.) used during the procedure for the preparation of thealginate solution and during the filtration steps are treated for 24 hwith an 1% etoxate solution (E-Taxa-Clean®, Catalog Number E9029.SigmaAldrich, Milan, Italy), then accurately washed with deionized waterand lastly autoclaved at 120° C. for 1 hr. The 30, 60 and 90pharmaceutical grade cellulose filters, all with 0.2 μm in pore size,and the cartridge with charge-modified Nylon 66 filtration membrane, 0.2μm pore size, are autoclaved, separately from the Housing, at 120° C.for 1 h.

Preparation of 1.8% AG Solution

AG (the sodium salt of alginic acid (E400)) used is Ketone® LVCR (Kelco)having a low degree of viscosity and provided in form of ultrapurepowder by the producer: Monsanto-Kelco (20 N Wacker Dr, Chicago Ill.USA). The chemical composition of alginate powder is described in termsof fractions (FG or FM) and MIG ratio, and the alginate used by thepresent inventors has an M percent of 52.26% and a G percent of 47.74%and the M/G ratio is 1.093, determined through NMR (Nuclear MagneticResonance) analysis. All the procedure for the preparation of the 1.8%alginate solution is carried out under a class II laminar-flowbiological hood. After weighing, the alginate powder is placed in abeaker and physiological solution (0.9% NaCl) is added slowly, to avoidclotting (the physiological solution used is sterile, apyrogenic andspecific for injectable preparations) under bland stirring attained byuse of a magnetic stirrer and a magnetic anchor, until obtaining ahomogeneous solution.

Filtration Systems (Housing) Assembly

At the moment of use, the suitable filter is inserted in the housing,all the procedure is carried out under biological hood, and said filteris closed and assembled on the specific support. Silicone tubes arefastened at the outlet and inlet of the housing. The tube at the inletof the filtration apparatus is hooked to a peristaltic pump, whereas thefree end is immersed in the beaker containing the alginate solution. Thefree end of the silicone tube at the outlet of the filtration system isplaced in a sterile collection bottle.

Filtration

The solution is subjected to 4 different filtration steps, which are allperformed by using the “Housing” filtration system and without breaks.

The first step provides for the solution to be filtered through acapsule of 30 pharmaceutical grade cellulose fiber (nominal pore size2.4 μm). By means of the peristaltic pump, the housing is filled withabout 7 liters of product, and filtrate collection is started. Pump rateis set at very low values, such as to allow a greater interactionbetween material and filter, and in fact the pressure inside the housingis held in the neighborhood of values of 1.5 bar. The first fraction ofthe filtrate, roughly quantifiable in 2 liters, is discarded as rich inextractables. The remainder is collected in plastic bottles, sterile andcertified for endotoxin absence.

The second filtration step provides for the use of a 60 pharmaceuticalgrade cellulose fiber capsule (nominal pore size 2 μm). The rate atwhich the pump is set is, in this case as well, such as to generate inthe filtration system a pressure of 1.5 bar. Collection occurs in thesame type of plastic bottles described above.

The third step on cellulose fiber capsule provides for the use of a 90pharmaceutical grade capsule (nominal pore size 2 μm). The rate at whichthe pump is set is, in this case as well, such as to generate in thefiltration system a pressure of 1.5 bar. Collection occurs in the sametype of plastic bottles described above.

The fourth and last filtration step provides for a cartridge with acharge-modified Nylon 66 filtration membrane, pore size 0.2 μm. In thislast case the pressure generated by the peristaltic pump is held in theneighborhood of 0.3-0.5 bar, by acting on the control of the rate of thepump itself. The filtrate is now collected in glass bottles pretreatedfor endotoxins and sterilized in autoclave at 120° C. for 1 h.

End Product Evaluation Example 2

Aliquots of the obtained product are tested for: endotoxin presence, bytaking a suitable aliquot and sending it to a company specialized inendotoxin dosage by Limulus method (Lonza Verviers, SPRL), for proteincontent by Bradford method, for pH value at +4° C. and at +20° C., usingthe micrometric method, and to confirm its chemical composition and therelated monomer fractions by NMR analysis. Endotoxin content is foundlower than 20 EU/g. Heavy metal presence and product sterility areassessed through standard protocols.

The present invention finds application in the field of transplantationbiotechnologies. In particular, in the case of the present inventors, AGmicrocapsules suitably coated with polyamino acids and diluted AG weredemonstrated to immunoprotect cells from the immune system of thereceiver. For several years now the inventors have been studying intheir laboratory the transplant of microencapsulated pancreatic islandsfor therapy of type 1 (insulin-dependent) diabetes mellitus and thisresearch activity is amply documented by scientific publications, mainlyinternational ones. Moreover, thanks to the purity and highbiocompatibility of the materials employed, the present inventors havebeen authorized by the Istituto Superiore di Sanita to operatively begina phase I clinical study related to transplant of microencapsulatedhuman pancreatic islands in diabetic receivers, not pharmacologicallyimmunosuppressed. From results obtained hereto, it emerges that themicrocapsules of the present inventors are highly biocompatible, and theeffectiveness of their product has been assessed in variousinternational research laboratories.

Capsules and artificial extracellular matrices for growth anddifferentiation of various cellular strains STANDARD OPERATIVE PROCEDURE(SOP) for the preparation of alginate/polyornithine (AG/PLO)microcapsules using sodium alginate produced according to the presentinvention

Reagents

-   -   sterile and apyrogenic physiological solution;    -   1.2% CaCl₂ solution in distilled water;    -   solution of 55 mM Na-citrate in distilled water;    -   0.12% and 0.06% polyornithine solutions in physiological        solution (the above-mentioned solutions are sterilized by        filtration)    -   1.6% NAG, obtained by the aforedisclosed filtration process;    -   0.05% NAG, by 1:10 dilution of the preceding one in        physiological solution.

Method

Islands are washed with physiological solution to remove any proteinpresent.

Then, for each ml of pellet 0.5 ml of physiological solution and 10 mlof 1.6% NAG are added, bringing the suspension to homogeneity.

The peristaltic pump is adjusted at 15 ml/min and the air flow at 5l/min, initially letting physiological solution flow through the system.

In a 250 ml beaker, containing 200 ml of 12% CaCl₂, there will becollected the alginate microdrops which, by gelling, will form theisland-containing alginate microcapsules. The distance of the needlefrom the meniscus of the CaCl₂ solution, equal to about 3 cm, assumescritical importance. Capsules are left S min in CaCl₂, then 100 ml of1.2% CaCl₂ are replaced with physiological solution and left again for Smin. Then, after repeated washings with physiological solution, thecapsules are transferred into a 50 ml falcon. Then, the followingreactants will be added in sequence, in an amount equal to twice thevolume taken up by the capsules, removing each time what was previouslyadded, stirring and carrying out between a reactant and the othersuitable washings with physiological solution:

-   -   0.12% polyornithine for 10 min;    -   0.06% polyornithine for 5 min;    -   0.1% NAG for 6 min;    -   55 mM Na-citrate for 2 min.

The repeated washings carry out a two-fold function, as they allow onthe one hand the removal of reactants or cellular debris, and on theother hand the removal of most of the smaller capsules, empty or broken,which by being lighter in weight settle more slowly. Upon ending thesetreatments, the capsules will be resuspended in CMRL 1066 medium. Priorto implant the capsules are washed with physiological solution, in whichthey will be resuspended also at the moment of implant.

1. A method for obtaining solutions of non-structurally modified saltsof alginate, with endotoxin content less than 20 EU/g, comprising: a.obtaining an alginate solution having a concentration ranging from 1.6to 2.0% by weight by adding commercial grade alginate powder to a salinesolution while controlling pH; b. filtering the alginate solutionobtained from step a) on at least one hydrophilic filter and recoveringa filtered alginate solution; and c. filtering the filtered alginatesolution obtained from step b) on a charge-modified hydrophobic filterand recovering an alginate solution having an endotoxin content lessthan 20 EU/g.
 2. The method according to claim 1, wherein said alginatepowder is sodium alginate powder.
 3. The method according to claim 1,wherein said alginate has a composition comprising 52.26% M and 47.74%G, corresponding to an M/G ratio of 1.093.
 4. The method according toclaim 1, wherein said step of filtering on at least one hydrophilicfilter is carried out on three hydrophil filters of cellulose acetatehydrophils.
 5. The method according to claim 5, wherein the first one ofsaid three filters has a pharmaceutical grade 30 and a nominal pore sizeof 2 μm, the second one of said three filters has a pharmaceutical grade60 and a nominal pore size of 2 μm and the third one of said threefilters has a pharmaceutical grade 90 and a nominal pore size of 2 μm.6. The method according to claim 1, wherein said hydrophobic filter is acharge-modified Nylon filter.
 7. The method according to claim 7,wherein said hydrophobic filter is a Nylon 66 filter having a positiveelectric charge.
 8. The method according to claim 1, wherein the salinesolution of alginate includes sodium alginate.
 9. The method accordingto claim 8, wherein the sodium alginate is employed for parenteral usein human transplants.
 10. The method according to claim 9, wherein thesodium alginate is formed into alginate/polyornithine microcapsules tobe used in human cell transplants.